Method of growing single crystals



United States Patent 3,150,925 METHQD 6F GRCWING SWGLE CRYSTALS Richard J. Garnbino, Middlefield, Conn, assignor to the United States of America as represented by the Secretar-y at the Army No Drawing. Filed Apr. 20, 1961, Ser. No. 104,433

2 Claims. (Cl. 23-50) (Granted under Title 35, US. Code (1052), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to a method fro growing single crystals of compounds of chemical formula B azMezFelgogg, B aMC Fe1 027 and Ba Me Fe O wherein Me is at least one divalent metal cation selected from the group Mg, Zn, Co, Ni, or Fe.

An object of this invention is to grow single crystals of the above-identified compounds that cm be recovered by a simple chemical treatment. Another object is to grow such single crystals at a melting temperature sufficiently low to eliminate the problem of the reduction of Fe+ thus improving the electronic properties of the product. A further object is to grow the single crystals at a temperature that greatly reduces the loss of volatile components. Another object is to grow the single crystals in such a manner that contamination from the vessels carrying the reactants, as for example crucibles, is negligible.

The group of compounds prepared by this invention are magnetic oxides of barium, iron, and at least one small divalent metal cation selected from the group Mg, Zn, Co, Ni, or Pe The generalized chemical formula and symbols of these compounds are as follows.

Formula: Symbol BZI MCgFCmOzg Y or M2Y BaM F1 O 7 Z 01 M6221 Ba Me Fe O W or NIeQW where Me represents Mg, Zn, Co, Ni, or Pe or combinations thereof. These compounds have properties which make them potentially useful as magnetic components for applications at microwave frequencies. In general, single crystals have an advantage over polycrystalline materials for microwave applications in the electromagnetic losses of single crystals are usually lower, that is, have a narrower microwave resonance line width. In addition, single crystals are important as standards by which to evaluate the performance of polycrystalline materials of the same composition.

7 Single crystals of the above group of compounds have een grown heretofore from melts of the component oxides. This method has a number of serious disadvantages. The crystals thus grown consist of inter-grown aggregates of all three groups of compounds as well as barium ferrite (BaFe C which can only be separated by tedious mechanical grinding and cleaving. The temperatures required to melt the mixed component oxides are in th order of 1600 C. so that contamination from the crucible, volatilization of certain constituents, and decomposition of other components are serious difliculties. The loss of oxygen from the melt by the decomposition of Fe O is especially serious since it is known that those compounds containing Fe+ have much higher microwave losses than those in which all of the iron is trivalent.

The method of this invention overcomes the aforementioned difiiculties in growing single crystals of these compounds. Briefly, the method includes dissolving a mixture of BaO, Met), where Me is at least one divalent metal 'ice cation selected from the group Mg, Zn, Co, Ni, or Pe and Fe O in Na CO at a temperature of about 1300 C., crystallizing the compounds by reducing the temperature 1-5 C. per hour to between 1000 C. and 1050 C., and

5 recovering the crystals by treating the solidified melt with hot, dilute nitric acid solution.

EXAMPLE 1 Ba Zn Fe O single crystals are grown from a mixture with the composition:

Compound Mole Ratio Weight in Grams HOEION The constituents are dry or wet mixed until homogene ous in appearance and firmly packed into a 30 cc. standard form platinum crucible which is covered. The crucible is heated to 1300 C. in an electric, resistance, furnace, slowly cooled at a rate of 2.5 C. per hour to 1060 C. at which temperature the power to the furnace is turned oil, and the furnace allowed to cool at its own rate (circa 100 C. per hour) to room temperature. The crucible and its contents are then placed in a beaker of nitric acid solution (10% by volume) and the solution is heated almost to boiling on a hot plate for several days. The acid solution is replaced as required to maintain a constant level. Most of the crystals are freed by this treatment; those which are not may be released by gently prying with a spatula. The yield of usable crystals is about 30% of the weight of BaO, Fe O and ZnO in the initial mixture.

EXAMPLE 2 Ba Mg Fe O single crystals are grown from a mixture with the composition:

Compound Mole Ratio Weight in Grams The ingredients are dry or wet mixed with a spatula and packed into a platinum crucible. The crucible is covered. The sample is then heated to 1240 C. in a 4 LD. Kanthal wound furnace, slowly cooled at a rate of 4.5 0/ hour to 1020 C. by means of a West Instrument J.S.G. Controller, then furnace cooled to room temperature. The batch is treated with hot (20% by volume) nitric acid for several days in order to free the crystals. Black hexagonal crystals result which are identified by X-ray diffraction analysis as single crystals corresponding [0 Mg Y.

When Example 2 above is repeated, however, using a mole ratio of 4 moles of Na CO instead of 2 moles, Mg Y crystals are not obtained.

EXAMPLE 3 Ba Zn Fe O single crystals are grown from a mixture with the composition:

The ingredients are dry mixed with a spatula and packed into a platinum crucible. The crucible is covered and the mix heated to 1257 C., slowly cooled to 930 C. at a rate of 45 C./hour, then furnace cooled to 540 C. and removed from the furnace. Single crystals corresponding to Zu W as determined by X-ray analysis are obtained.

Single crystals of these compounds containing more than one divalent metal ion selected from the group Mg, Zn, Co, Ni, or Fe+ may also be grown by this method. By varying the proportions of divalent metal oxides in the starting composition, it is possible to vary the proportion of the two or more divalent metal ions in the product crystals. Since certain electronic properties of the compound depend on the proportions of divalent metal cations, for example, the microwave resonance frequency, it is possible to control these properties in the product crystals by controlling the composition of the starting mixture.

Other examples showing conditions for growing crystals and results obtained are summarized in the following table. In the table, column A represents the single crystal compound prepared; columns B, C, D, and E represent the mole ratios of Na CO BaCO Fe O and MeO, respectively, used in preparing the single crystal compound; column F, the maximum temperature of the run in degrees centigrade; and column G, the cooling rate in de grees centigrade per hour.

Table In the method, the concentration of Na CO flux may be between 5 and mole percent of the total molar amounts of reactants and flux used. As alkali metal oxides tend to attack platinum at elevated temperatures in oxidizing atmosphere, the use of the low flux concentrations in this method is especially advantageous. That is, it has been found that the attack on the platinum crucibles used in the invention is negligible.

An especially desirable embodiment of this invention is the growing of the single crystals of the Mg Y type. These crystals are grown by dissolving a mixture of BaO, MeO, and Fe O in Na CO at about 1300 C. and then crystallizing the material by gradually reducing the temperature. The crystals are then recovered by treating the solidified melt with hot, dilute nitric acid solution, thereby freeing the crystals from the matrix. Carbonates or other compounds which decompose to the appropriate oxides at the temperatures involved may be used in place of the oxides. The fluxingphase in the melt has been found to be sodium ferrite (Na FeO so that other so- A dium compounds may be used in the initial mixture, as for example, Na O, NaOH or Na Fe O instead of Na CO The single crystals grown by this invention are of high perfection and reduce the ferrimagnetic resonance line width. For most microwave applications of ferrimagnetic materials, the characteristics of devices improve as the line width decreases.

It is intended that the foregoing description be considered illustrative only and not in limitation of the invention as hereinafter claimed.

What is claimed is:

1. The method of growing single crystals of compounds of chemical formula Ba,Me,Fe, o BaM2F1 O27, and Ba Me Fe O wherein Me is at least one divalent metal cation taken from the group consisting of Mg, Zn, Co, Ni, and Fe, comprising dissolving a mixture of BaO, MeO where Me is at least one divalent metal cation selected from the group consisting of Mg, Zn, Co, Ni, and Fe+ and Fe O in sodium carbonate in a covered platinum crucible in an electric resistance furnace at a temperature of about 1300 C., the concentration of sodium carbonate being between 5' and 15 mole percent of the total molar amounts of materials used and crystallizing the compounds by reducing the temperature l-5 C. per hour to between 1000 and 1050 C., and then recovering the crystals by treating the solidified melt with 10 percent to 20 percent by volume nitric acid solution.

2. The method of growing single crystals of compounds of chemical formula Ba Me Fe O wherein Me is at least one divalent metal cation taken from the group consisting of Mg, Zn, Co, Ni, and Fe, comprising dissolving a mixture of BaO, MeO, where Me is at least one clivalent metal cation selected from the group consisting of Mg, Zn, Co, Ni, and Fe, and Fe O in sodium carbonate in a covered platinum crucible in an electric resistance furnace at a temperature of about 1300 C., the concentration of sodium carbonate being between 5 and 15 mole percent of the total molar amounts of materials used and crystallizing the compounds by reducing the temperature 1-5 C. per hour to between 1000 and 1050 C., and then recovering the crystals by treating the solidified melt with 10 percent to 20 percent by volume nitric acid solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,837,483 Hakker et al June 3, 1958 2,854,412 Brockman et al. Sept. 30, 1958 2,955,085 Jonker et al Oct. 4, 1960 2,977,312 Gortes et al Mar. 28, 1961 3,062,746 MacCallum Nov. 6, 1962 3,079,240 Remeika Feb. 26, 1963 3,102,099 Stuijts Aug. 27, 1963 FOREIGN PATENTS 324,068 Switzerland Oct. 15, 1957 OTHER REFERENCES Mones et -al.: J. Phys. Chem. Solids, Pergamon Press, 1958, vol. 4, pages 217 to 22-2.

Jonker et al.: Philips Technical Review," vol. 18, No. 6, November 1956, pages -180. 

1. THE METHOD OF GROWING SINGLE CRYSTALS OF COMPOUNDS OF CHEMICAL FORMULA BA2ME2FE12O22, BAME2FE16O27, AND BA3ME2FE21O41, WHEREIN ME IS AT LEAST ONE DIVALENT METAL CATION TAKEN FROM THE GROUP CONSISTING OF MG, ZU, CO, NI, AND FE+2, AND FE2/3 IN SODIUM CARBONATE IN A COVERED PLATINUM CRUCIBLE IN AN ELECTRIC RESISTANCE FURNACE AT A TEMPERATURE OF ABOUT 1300*C., THE CONCENTRATION OF SODIUM CARBONATE BEING BETWEEN 5 AND 15 MOLE PERCENT OF THE TOTAL MOLAR AMOUNTS OF MATERIALS USED AND CRYSTALLIZING THE COMPOUNDS BY REDUCING THE TEMPERATURE 1*-5*C. PER HOUR TO BETWEN 1000 AND 1050*C., AND THEN RECOVERING THE CRYSTALS BY TREATING THE SOLIDIFIED MELT WITH 10 PERCENT TO 20 PERCENT BY VOLUME NITRIC ACID SOLUTION. 